The present invention relates to a thermal type flow measuring apparatus for measuring the flow of fluid, and more particularly to a heat-resistant thermal type flow measuring apparatus used for the air flow sensor of an internal combustion engine or the air flow sensor of a fuel cell system.
Conventionally, a thermal type air flow sensor, capable of directly sensing the mass air flow, has been widely used as an air flow sensor in the intake air path of an internal combustion engine, for example, in a car, for measuring the intake air flow. Recently, an air flow sensor, manufactured based on the semiconductor micro-machining technology, has attracted special attention because it has fast response that allows even a backflow to be sensed.
One of known thermal-type air flow sensors is a sensor that heat controls a thermal resistor to measure the air flow based on the heat radiation amount of the heating resistor. Another known thermal-type air flow sensor is a heat-resistant thermal type air flow meter that heat controls a heating resistor to measure the air flow according to a change in the temperature of a temperature-sensing resistor placed near the heating resistor.
The problem with air flow detection by means of this heating resistor is that the resistance value of the heating resistor deteriorates with age and the change in the resistance value changes the sensitivity characteristics that, in turn, changes the air flow signal value.
A method is proposed in JP-B-6-63801 to solve this problem. According to the proposed method, a correction circuit is provided to correct a change in the air flow characteristics, caused by a change in the resistance value of the heating resistor, to prevent the output air flow value from being affected by a change in the characteristics of the heater resistor.